Quirks Uncovered While Quirks Uncovered While Testing Forensic ToolTesting Forensic Tool

Quirks Uncovered While Quirks Uncovered While Testing Forensic ToolTesting Forensic Tool

Jim Lyle

Information Technology Laboratory

Agora

March 28, 2008

Agora, Seattle 2

DISCLAIMERDISCLAIMER

Certain trade names and company products are mentioned in the text or identified. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the products are necessarily the best available for the purpose.

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OutlineOutline

Overview of computer forensics at NISTQuirks uncovered

– Write Blocking– Acquisition to an image file– Restoration from an image file– Other

Questions and answers

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Where is CFTT?Where is CFTT?

US government, executive branch Department of Commerce (DOC) National Institute of Standards and Technology

(NIST) Information Technology Lab (ITL) Software Diagnostics and Conformance Testing

Division (SDCT) Computer Forensics: Tool Testing Project (CFTT) Also, the Office of Law Enforcement Standards

(OLES) at NIST provides project input

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Goals of CF at NIST/ITLGoals of CF at NIST/ITL

Establish methodology for testing computer forensic tools (CFTT)

Provide international standard reference data that tool makers and investigators can use in investigations (NSRL, CFReDS)

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Project Sponsors (aka Project Sponsors (aka Steering Committee)Steering Committee)

National Institute of Justice (Major funding) FBI (Additional funding) Department of Defense, DCCI (Equipment and

support) Homeland Security (Technical input) State & Local agencies (Technical input) Internal Revenue, IRS (Technical input) NIST/OLES (Program management)

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Other Related Projects at Other Related Projects at NISTNIST

NSRL -- Hash (MD5, SHA1) file signature data base, updated 4 times a year (Doug White, John Tebbutt, Ben Long)

PDAs and Cell Phones, NIST (Rick Ayers)SAMATE -- Software Assurance Metrics

and Tool Evaluation (Paul E. Black)CFReDS -- Computer Forensics Reference

Data Sets (Jim Lyle)

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Forensic Tool FeaturesForensic Tool Features

… are like a Swiss army knife– Blade knife for cutting– Punch for making holes– Scissors for cutting paper– Cork screw for opening Chianti

Forensic tools can do one or more of …– Image a disk (digital data acquisition)– Search for strings– Recover deleted files

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Testing a Swiss Army KnifeTesting a Swiss Army Knife

How should tools with a variable set of features be tested? All together or by features?

Test by feature has a set of tests for each feature: acquisition, searching, recovery

Examples: EnCase acquisition, iLook string search, FTK file recovery

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Good NewsGood News

Forensic tools tested workProblems found are minor

– Usually something is omitted– Nothing incriminating is created

Investigators should be aware of the quirks

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Write BlockingWrite Blocking

Goal: Prevent changes to a protected driveHost interacts with a drive by a command

set through an interface– Read– Write– Control & info

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Int 13 Extended WriteInt 13 Extended Write

DOS Interrupt 13 has three write cmds– Write (original write cmd)– Write long– Extended write (added later for large drives)

Early write blocker versions only block write & write long

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Blocking read commandsBlocking read commands

Hardware write block devices …– Capture cmds sent from a host on a bus– Send cmds to a protected device– Return data to a host

Some devices may …– Substitute a different cmd– Cache results and not issue cmd to device. If the

protected device is reconfigured to report a different size, a cached size is reported incorrectly

– Block some read cmds

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Allow Reads vs Block WritesAllow Reads vs Block Writes

Block unsafe commands, allow everything else+ Always can read, even if new command introduced- Allows newly introduced write commands

Allow safe commands, block everything else+ Writes always blocked

- Cannot use newly introduced read commands

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Source AcquisitionSource Acquisition

Tool acquires either – entire drive (physical drive) – partition (logical drive)

Evaluate the acquisition by either …– Hash of data acquired– Compare source to a restore

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Hard Drive OrganizationHard Drive Organization

Basic unit is a 512 byte sector63 sectors are grouped as a track A set of tracks are grouped as a cylinderSectors are addressed as:

Cylinder/track/sector

Cylinders start at 0,

Tracks start at 0, but

Sectors start at 1

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Odd SectorsOdd Sectors

Use dd to acquire either a physical or logical drive with an odd sector count and the last sector is omitted.

Occurs in the 2.4 kernel and earlier.The current 2.6 kernel does not have the

problem.

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BIOS LiesBIOS Lies

DOS based acquisition via BIOS interface Some BIOSs group several physical cylinders

together into a logical cylinder There may be a fractional logical cylinder left

over. In addition, some BIOSs may underreport the

number of logical cylinders by 1 cylinder

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More BIOS LiesMore BIOS Lies

Actual geometry: 3,309/16/63– 63x16 sectors/cylinder (1,008)– 3,335,472 total sectors 3,335,472x512 bytes

BIOS wants cylinder count < 1024BIOS reports geometry as: 826/64/63

– 63x16 sectors/cylinder (4,032)– 3,330,432 total sectors

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Restoring an ImageRestoring an Image

Testing the accuracy of a restore …Compare the original source sector by

sector to the restored image

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Missing Sectors on RestoreMissing Sectors on Restore

Restore an image of an IBM-DTLA-307020 with 40188960 sectors to an identical drive the results are …

Sectors Compared 40188960 Sectors Differ 10395 Diffs range 40178565-40188959

Also the partition table gives 255 heads/cylinder and 63 sectors/track.

That gives 16,065 (63*255) sectors/cylinder Note that 40,188,960 mod 16,065 is … 10,395March 28, 2008

Next Quirk, Starting with Next Quirk, Starting with AnswerAnswer

AAAAA BBBBB CCCCC DDDDD EEEEE

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AAAAA BBBBB CCCCC BBBBB

A 27,744,120 sectorsB 7 sectorsC 57 sectorsD 7 sectorsE 1 sector

Image an NTFS partition of 27,744,192 sectors

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NTFS Partition RestoreNTFS Partition Restore

Setup NTFS partition– MD5: 92b27b30bee8b0ffba8c660fa1590d49– 27,744,192 sectors– Each sector filled with sector LBA & disk ID

Acquire partition– Total Sectors:27,744,191– 494A6ED8A827AD9B5403E0CC89379956

Rehash (minus last sector) -- still no match

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More NTFSMore NTFS

Restore image to NTFS partitionCompare to original

– Sectors differ: 47Restore was in Windows XP … Restore again, unpower drive, no system

shutdown. Compare to original– Sectors differ: 8– Diffs range: 27,744,184-27,744,191

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NTFS ResolutionNTFS Resolution Examine the eight sectors

– Last sector not imaged– Other seven are a second copy of seven sectors starting at offset 27,744,120 --

Know this because each sector is tagged with LBA Verification:

Acquisition hash: 494a6ed8a827ad9b5403e0cc89379956xena:/Users/jimmy root# dd bs=512 if=/dev/disk2s11

of=~jimmy/nt.dd xena.local(1009)==> dd if=nt.dd bs=512 skip=27744120 count=7

of=end.ddxena.local(1012)==> dd if=nt.dd bs=512 count=27744184 of=chunk.ddxena.local(1013)==> cat chunk.dd end.dd | md5494a6ed8a827ad9b5403e0cc89379956xena.local(1022)==> md5 nt.ddMD5 (nt.dd) = 92b27b30bee8b0ffba8c660fa1590d49

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Faulty Sector BehaviorsFaulty Sector Behaviors

Some sectors adjacent to faulty sector missed– ATA interface: 8 sector window– USB interface: variable size window < 64– FW interface: variable, but different from USB

Missed sectors filled with unknown dataImage file gets out of sync

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Imaging a Drive with Faulty Imaging a Drive with Faulty SectorsSectors

Acquire all sectors that are not faulty,identify all faulty sectors, andin the image file replace the faulty sector

content with benign fill.

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Reliably Faulty DrivesReliably Faulty Drives

A set of known consistently faulty sectors.Can be imaged repeatedly with the same set

of sectors reporting failure.Set of three reliably faulty drives:

– MAX1 (54 faulty sectors)– MAX2 (398 faulty sectors)– WD (22 faulty sectors)

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Basic Imaging ToolsBasic Imaging Tools

DCCIdd V 2.0DCFLdd V 1.3.4dd on Helix with Linux kernel 2.6.14dd on FreeBSD V 5.5IXimager V 2.0 February 1, 2006

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MethodologyMethodology

1. Create a reference drive identical to the faulty drive, but with no faulty sectors.

2. Clone the faulty drive with an imaging tool.

3. Compare the clone to the reference drive.

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Results for Drive MAX1Results for Drive MAX1

Tool Bus readable sectors missed

IXimager FW 0

Helix dd FW 5034

DCFLdd FW 5034

DCCIdd ATA 306

FreeBSD dd FW 0

The missed sectors were misidentified as faulty and filled with zeros.

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DCCIdd ATA InterfaceDCCIdd ATA Interface

Look at first difference between the clone and reference drive.

The first difference is a run of 8 sectors, all zeros, on the clone (10,069,088 - 10,069,095).

First faulty sector at address 10,069,095.DCCIdd misidentifies seven sectors as

faulty on messages to stderr.

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More Runs (ATA Interface)More Runs (ATA Interface)Next four runs …

Bad Sector Run Start Run End

10069911 10069904 10069911

12023808 12023808 12023815

18652592 18652592 18652599

18656041 18656040 18656047

•All runs included at least on faulty sector.

•All runs were 8 sectors long.

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DCFLdd, dd & FirewireDCFLdd, dd & Firewire

Some sectors around a faulty sector misidentified as faulty and imaged as zeros.

Unlike ATA, the length of the run of misidentified sectors including the faulty sector varied.

First five run lengths: 168, 216, 72, 248, 112. Note: all are a multiple of 8. Faulty sector was always in last group of 8.

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Results: Sectors MissedResults: Sectors Missed

For IXimager and FreeBSD dd all the run lengths are one (no readable sectors missed).

For imaging directly to the ATA interface with dd based tools the run length for a single isolated faulty sector was eight sectors (with seven sectors misidentified as faulty).

For imaging with dd over the Firewire interface, the run lengths associated with a single, isolated faulty sector were a multiple of eight sectors (also with readable sectors misidentified as faulty).

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Results: Fill ContentResults: Fill Content

IXimager filled the sectors with the string: ILookImager_Bad_Sector_No_Data

All the tools running in the Linux environment filled the sectors with zeros (NULL bytes).

The sectors created by dd running in FreeBSD contained data from an undetermined source.

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Other QuirksOther Quirks

Hash Quirks– Screen hash differ from log file– Multiple hashes: SHA ok, MD5 wrong;

hardware dependent

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ContactsContactsJim Lyle Doug White

www.cftt.nist.gov www.nsrl.nist.gov

cftt@nist.gov nsrl@nist.gov

Sue Ballou, Office of Law Enforcement Standards

Steering Committee Rep. For State/Local Law Enforcement

susan.ballou@nist.gov

March 28, 2008